1,720,999 research outputs found
ESA CAMELOT study : Challenges in future operational missions for GMES atmospheric monitoring, sentinel 4 and 5
No full textNo abstract
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Satellite observations of ozone and nitrogen dioxide : from retrievals to emission estimates
Get PDFIn the last decades, measurements of atmospheric composition from satellites have become very important for scientific research as well as applications for monitoring and forecasting the state of the atmosphere. Instruments such as GOME-2, and OMI look at backscattered sunlight in nadir view, measuring the ultraviolet and visible spectrum in high resolution. Launched in a sun-synchronous orbit at ??800 km altitude, they scan the Earth’s surface daily in 14–15 orbits, providing a homogeneous dataset with (almost) daily global coverage. Combining the spectral measurements with radiative transport models, concentrations can be inferred for important trace gases such as ozone (O3) and nitrogen dioxide (NO2). Chemical transport models can be used to calculate the strength and location of the underlying emissions. Long time series of satellite retrievals give insight on how human activity contributes to changes of atmospheric composition, affecting health and climate. Information in the vertical distribution of ozone can be retrieved from the sharp decrease in the ozone absorption cross-section in the ultraviolet spectrum. Chapter 2 deals with the question how the performance of the ozone profile retrieval algorithm (OPERA) can be improved. To produce consistent global datasets, the algorithm needs to have good global performance, while short computation time facilitates the use of the algorithm in near real time applications. Because the retrieval is ill-posed (in the sense that many profiles give similar simulated spectra within the measurement errors), the solution depends on a priori (climatological) ozone profiles. The non-linearity of the problem asks for an iteration scheme to find the best fitting solution numerically. We use the convergence behaviour of the iteration as a diagnostic tool for the ozone profile retrievals from the GOME instrument for February and October 1998. In this way, we reveal several retrieval problems of different origin, and we improve issues related to the Southern Atlantic Anomaly, low cloud fractions e.g. above deserts, and ozone cross sections. The a priori ozone climatology and its associated variability is also an important source for retrieval problems. By using a priori ozone profiles that are selected on the expected total ozone column, retrieval problems due to anomalous ozone distributions (such as in the ozone hole) can be avoided. Applying the algorithm adaptations improve the convergence statistics considerably, not only increasing the number of successful retrievals, but also reducing the average computation time, due to less iteration steps per retrieval. For February 1998, non-convergence was brought down from 10.7% to 2.1%, while the mean number of iteration steps (which dominates the computational time) dropped 26% from 5.11 to 3.79. Total nitrogen dioxide columns can be retrieved from space in the 405–465 nm window, but the NO2 spectrum does not contain any significant height information. Instead, data assimilation techniques can be used to distinguish the tropospheric part from the stratospheric part, which gives valuable information of NO2 in the lowest part of the atmosphere. Here it acts as an air pollutant, often from man-made origin. The case study in Chapter 3 evaluates how NO2 air pollution can be controlled with air quality measures. Due to strong economic growth in the last decades, air pollution in large Chinese megacities has become a serious issue. In reparation for the Olympic Games in Beijing in 2008, extensive air quality measures were taken to improve air quality during the event, affecting traffic, industry and power production. We evaluate the effect of the air quality measures on reducing air pollution, by analysing the tropospheric NO2 retrievals over the greater Beijing area before, during and after the Olympic Games. To compensate for the strong variability due to meteorology, we compare the observations with model simulations from the regional chemistry transport model CHIMERE based on a pre-Olympic emission inventory. The relative change between observation and simulation shows that the measures caused a reduction of tropospheric NO2 column concentrations of approximately 60% above Beijing during the Olympic period. The air quality measures were especially effective in the Beijing area, but also noticeable in surrounding cities of Tianjin (30% reduction) and Shijiazhuang (20% reduction). In the months after the Olympic events, NOx emissions in Beijing show a slow recovery towards pre-Olympic levels. In a next step, we use the difference between NO2 observations and simulations to adjust the emission inventory used by the model. Emission inventories of air pollutants are crucial information for policy makers and form important input data for air quality models. Chapter 4 presents a new algorithm specifically designed to use daily satellite observations of column concentrations for fast updates of emission estimates of short-lived atmospheric constituents on a mesoscopic scale (??25??25 km2). The algorithm needs only one forward model run from a chemical transport model to calculate the sensitivity of concentration to emission, using trajectory analysis to account for transport away from the source. By using a Kalman filter in the inverse step, optimal use of the a priori knowledge and the newly observed data is made. We apply the algorithm for NOx emission estimates of East China, using the CHIMERE model on a 0.25 degree resolution together with tropospheric NO2 column retrievals of the OMI and GOME-2 satellite instruments. Closed loop tests show that the algorithm is capable of reproducing new emission scenarios. Applied with real satellite data, the algorithm is able to detect emerging sources (e.g. new power plants), and improves emission information for areas where proxy data are not or badly known (e.g. shipping emissions). It is shown that chemical transport model runs with the daily updated emission estimates provide better spatial and temporal agreement between observed and simulated NO2 concentrations, which facilitates an improved air quality forecast for East China. Monthly emission estimates give valuable insight in changing biogenic and anthropogenic activity. In Chapter 5, the emission estimation algorithm is used to construct a monthly NOx emission time series for 2007–2010 from tropospheric NO2 observations of GOME-2 over East Asia. Most Chinese provinces show a strong positive trend during this period, related to the country’s economic development. Negative emission trends are found in Japan and South Korea, which can be attributed to a combined effect of local environmental policy and global economic crises. The algorithm is also used to quantify the direct effect of regional NOx emissions on tropospheric NO2 concentrations elsewhere. Due to transport of air pollution, high NOx emissions not only affect local air quality, but also contribute significantly to tropospheric NO2 in remote downwind areas
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Tropospheric nitrogen dioxide inversions based on spectral measurements of scattered sunlight
Get PDFThis thesis describes the development of inversion methods for tropospheric nitrogen dioxide (NO2), based on ground based observations of scattered sunlight with themulti-axis differential optical absorption spectroscopy (MAX-DOAS) technique. NO2 is an atmospheric trace gas which, when present near the surface, is an important component of air pollution. On a global scale, fossil fuel combustion processes (power plants, automobiles) are the main source of NOx (=NO+NO2). NOx has a major impact on air quality, by its essential role in atmospheric photochemistry mechanisms: it influences tropospheric ozone formation as well as the levels of other oxidants (such as the hydroxyl and peroxy radicals), which catalyze the removal of species like carbonmonoxide, methane and other hydrocarbons from the atmosphere. In addition, NOx affects the formation of aerosols. Through the combination of these effects, tropospheric NOx reduces the radiative forcing, and therefore has a cooling effect on climate. A MAX-DOAS instrument measures wavelength spectra in the UV/Vis, with a resolution of less than one nanometer, in multiple viewing directions relative to the horizon. These measurements are analyzed with the differential optical absorption spectroscopy (DOAS) method, which can distinguish between the traces gases absorbing in a certain spectral window by making use of the fact that each gas has a unique spectral fingerprint. MAX-DOAS type of observations are complementary in two ways to other measuring techniques for NO2: Firstly in a temporal sense, relative to space-borne observations from the current generation of polar orbiting satellites, which frequently have no more than one observation per day. Secondly the MAX-DOAS observations are complementary in a spatial sense to in-situ monitors, which can only measure NO2 at the surface. MAX-DOAS instruments are especially suitable to measure tropospheric NO2 columns. This quantity is, more than NO2 concentrations measured at the surface, relevant for studies of transport and of trends in total amounts of tropospheric NO2. In this work it is investigated which information about the total amount and vertical distribution of tropospheric NO2 is contained in the MAX-DOAS measurements, and how this information can be extracted through inversemodeling (retrieval). What are the main error sources and which assumptions have to be made? Special attention is paid to aerosols, which have a large impact on the MAX-DOAS measurements. In addition, the developed retrieval methods are applied to a 14 month data set of MAXDOAS measurements performed in De Bilt. This data set was obtained as part of this research, and is unique for the Netherlands. The results are compared to satellite observations and to an air quality model. The first research described in this thesis (Chapter 3), focuses on the retrieval of tropospheric NO2 columns under clear sky conditions. A method was developed to derive differential air mass factors, by taking into account the effect of aerosols on the NO2 measurements. This was done in a new way: it was demonstrated that the aerosol optical thickness could be derived, for each elevation separately, by solely making use of MAX-DOAS measurements of relative intensity. It was assumed that all NO2 and aerosols were contained in a homogeneously mixed boundary layer of 1 km height. With this method, aerosol corrected air mass factors were derived for the elevations 4 degrees, 8 degrees and 16 degrees. Vertical columns could be derived for each elevation separately. Within this set of elevations, the 4 degrees elevation has the advantage of being most sensitive to trace gases in the boundary layer, but this viewing direction is also most sensitive to errors in the assumed aerosol and NO2 profile shape. With increasing elevation, the sensitivity to traces gases decreases, as well as errors due to wrong assumptions about the profile shapes. The retrieval method was applied to clear sky periods within the 14 month data set of MAX-DOAS observations performed in De Bilt. A comparison with AErosol RObotic NETwork (AERONET) observations of aerosol optical thickness showed good results: correlation of 0.85 was found, and a slope of the linear fit close to one. Comparison with space-borne retrievals of tropospheric NO2 columns retrieved by the OzoneMonitoring Instrument (OMI) shows on average reasonable results (correlations between 0.64 and 0.88 for different subsets), but individual comparisons can differ by more than a factor of two. This was attributed for the largest part to differences in spatial representativity, mostly in the horizontal direction, but also in the vertical. The second study (Chapter 4) focused on the question which information about the vertical distribution of aerosols and NO2 is contained in the MAX-DOAS observations. Vertical profile information derived from MAX-DOAS observations is not only relevant to improve the accuracy of the column retrieval, but it is needed as well in comparisons with other measurement techniques, such as satellite and lidar, and to derive NO2 surface concentrations, which are more directly related to air quality than vertical columns. Profile retrieval is challenging, since the profile information contained in MAX-DOAS measurements is known to be quite low. The topic is currently an active area of research in the MAXDOAS community: a range of approaches is being investigated, and no approach has yet emerged that can be considered a proven concept in all respects, partly because validation is a challenge as well. One of the main research questions addressed in this study, was the question if MAX-DOAS measurements can be used to distinguish between NO2 in the boundary layer and in the free troposphere. The basic retrieval model for tropospheric columns of the first study was expanded with two parameters for NO2, and one for aerosols: the height of the aerosol and NO2 layer above the surface was not longer assumed �fixed, and a second elevated NO2 layer was introduced at a fixed altitude in the free troposphere. In addition O4 measurements were used instead of relative intensity measurements, in order to better characterize the aerosol extinction profile in the boundary layer. Sensitivity studies were performed to investigate the retrieval accuracy for different noise levels, and for aerosol and NO2 profile shapes that were different from those assumed in the retrieval model. This led to the following conclusions: Firstly, if (in reality) NO2 is present above the boundary layer, and if the retrieval model allows an elevated NO2 layer at the same altitude as the real layer, then the amount of elevated NO2 can in principle be retrieved with reasonable accuracy. Secondly, for retrieval models which allow an elevated NO2 layer, tropospheric NO2 may also be retrieved when it is not present in reality. This may be the case for example when the aerosol andNO2 profiles shapes in the boundary layer are not well described by the retrieval model, or when the signal to noise level is low. Finally, despite the fact that MAX-DOAS measurements frequently do not contain more than two or three pieces of information to describe the NO2 profile, a retrieval model with only three free parameters will frequently be too rigid to perform accurate retrievals. In Chapter 6 of this thesis a more flexible approach is proposed. The profile retrieval approach was applied to MAX-DOAS measurements taken at the Cabauw Intercomparison campaign for NItrogen Dioxide measuring Instruments (CINDI). Comparison with independent observations of tropospheric NO2 columns from a lidar and NO2 surface concentrations from an in-situ monitor, showed on average good agreement (an average difference below 5 percent for both), but significant differences for individual cases. The third part of the research (Chapter 5) consisted of a comparison of tropospheric NO2 columns derived from the 14 month data set (that was also used in the first study) with tropospheric columns from the regional air quality model Lotos-Euros, which was run on a resolution of approximately 7x7 km. Whereas the comparison with satellite observations (Chapter 3) could only be performed under cloud free conditions, and at most two times per day, the comparison with the air quality model could be performed by making use of all MAX-DOAS observations. The total data volume therefore increased by a factor of 30, which improved the statistical significance, and allowed more detailed case studies. In order to analyze MAX-DOAS measurements under cloudy conditions, it was required to develop a third retrieval approach. This was based on MAX-DOAS observations at 30 degrees elevation (and the zenith reference), in order to be least sensitive to errors in the assumed NO2 profile shape, and to aerosols (the retrieval of which is difficult under cloudy conditions). Air mass factors were derived using information about the boundary layer height from a meteorological model. In addition, lidar observations of cloud bottom height were used. The comparison between Lotos-Euros and MAX-DOAS showed on average a good agreement (an average difference below 1 percent, and for daily averages over cloud free days a correlation of 0.8). The agreement found was surprising, especially when considering the fact that a bottom-up approach (the model) is compared to a top-down approach (the measurements). Furthermore, a remarkably good agreement was found for the tropospheric NO2 column averaged per sector of the wind direction. This indicates that the average tropospheric NO2 column that is measured in De Bilt, is not dominated by local sources, such as nearby highways with frequent traffic jams (such emissions are difficult to capture in the model), but rather by emissions in densely populated and industrial areas further away, e.g. the cities of Rotterdam, Amsterdam, Antwerp, Brussels and the German Ruhr area. It appears that tropospheric NO2 columns measured with MAX-DOAS can be used for validation of (high resolution) chemistry transport models in urban regions, and the same may be expected for satellite observations with a sufficiently small resolution. This is especially relevant because it is known that comparison between satellite and in-situ is problematic in urban regions (due to the large difference in spatial representativity). No observations were performed in summer months. It may however be expected that because of the shorter lifetime of NO2 in summer, nearby sources would have a larger relative impact on the MAX-DOAS observations, and therefore lead to less agreement with the model than as found in this study. For individual comparisons on an hourly and day-to-day basis, observed differences could be substantial. This is mainly attributed to the fact that within the model actual emissions cannot be described on a high enough spatial and temporal resolution. In addition, differences could be large for example when the observed wind direction or wind speed was different from that in the model, or in the weekend: Observations showed on average a clear decrease in the weekend, compared to the rest of the week, whereas the model showed a less pronounced weekly cycle. The three studies described in this work lead to the following general conclusions about MAX-DOAS observations of NO2: Firstly, it has been demonstrated that long-term MAX-DOAS observations of tropospheric columns are particularly suitable for validation of space-borne observations and air quality models. When averaged over long-enough periods, patterns show up (e.g. as a function of time, wind direction, or another quantity) that would not be seen for individual comparisons, or not even for a few months of observations, due to differences in representativity and limited accuracy. Thorough satellite and model validation therefore requires a large network of MAX-DOAS sites, on locations with a variety of conditions with respect to NO2 and aerosols. There is currently no other ground based method that can provide automated tropospheric NO2 column observations for such a low cost per observation. Secondly, it is concluded that with a simple algorithm, based on a high viewing elevation (e.g. 30 degrees), tropospheric NO2 columns can be retrieved with reasonable accuracy, for a large number of different aerosol and NO2 scenarios, and without strong dependence on a-priori assumptions. With respect to NO2 profile retrieval, the situation is different: due to the many aspects that influence the measurements, the required accuracy, and the relatively low information content, the accuracy of individual retrievals is generally not very high (especially for the free troposphere), and depends strongly on a-priori assumptions, as well as on the atmospheric conditions at the time of measurement. To make optimal use of the information content, it is important that the profile shape parametrization is highly flexible for the lowest 1-2 kilometers of the atmosphere. Profile retrieval accuracy is highest and depends least on a-priori assumptions for cloud free situations, when the aerosol optical thickness is low, when the NO2 is located in a boundary layer of which the top lies between approximately 400 m and 1.5 km altitude, and when in addition the MAX-DOAS instrument is aimed away from the sun. Finally, it is concluded that more validation of MAX-DOAS retrieval methods is needed. This requires long-term observations in the presence of other instruments that can be used for comparisons, such as those present at the CINDI campaign. Such comparisons should be performed in various seasons and under various conditions with respect to the abundance of aerosols and NO2
On satellite observations of atmospheric composition and their interpretation
Get PDFSince more than 30 years satellites contribute significantly to our understanding of the composition of the atmosphere by performing global observations of atmospheric constituents from space. A recent addition to the series of Earth observing instruments is the Ozone Monitoring Instrument (OMI) that since October 2004 performs daily global measurements at high spatial resolution. The work presented in thesis focuses on spaceborne observations of NO2 and tropospheric aerosols, and the interpretation of the behavior of these constituents. NO2 plays an important role in the chemistry of the atmosphere due to its involvement in the catalytic destruction of ozone in the stratosphere, and by being a precursor of tropospheric ozone, linking NO2 to air quality and climate change. Aerosols also play an important role in chemistry and climate. For the DOAS-based retrieval of NO2 from OMI measurement data an accurate characterization of the OMI spectral slitfunction is essential. The spectral slitfunction was characterized with a novel method where the slitfunction for each wavelength and viewing angle was sampled by the spectrally narrow diffraction orders of an echelle grating, with wavelength increments 10 times smaller than the spectral resolution of OMI. The resulting parameterization of the spectral slitfunction is used in the retrieval of NO2 and other DOAS-based products from OMI. Tropospheric NO2 columns are retrieved from OMI measurements on an operational basis by the Dutch OMI NO2 (DOMINO) system. The DOMINO algorithm assimilates NO2 slant column in the TM4 chemistry transport model to estimate the stratospheric NO2 column. DOMINO data are available as a near-real time (within 3-4 hours after measurement) and as a consistent reprocessed offline dataset of collection 3, version 1.0.2. Based on the findings of validation studies involving DOMINO data, improvements to the DOMINO algorithm regarding surface albedo and a priori profile shape are identified . An extensive validation study shows that OMI stratospheric NO2 columns are consistent within 13% with ground-based observations from the SAOZ and NDACC network. The DOMINO product performs superior to the parallel existing Standard Product by capturing the dynamic variability of NO2 in the stratosphere, such as the daytime increase of stratospheric NO2 and the day-to-day variations in the NO2 field associated with the collapse of the Arctic Polar vortex. Analysis of the 5+ year OMI data record shows that OMI observes variations in stratospheric NO2 on a seasonal and multi-annual scale, e.g., the quasi biennial oscillation (QBO) and trends. The NO2 QBO signal exhibits a distinct interhemispheric asymmetry over the tropics, and is stronger over the Southern Hemisphere. There is good agreement between the Lauder data record and collocated OMI stratospheric NO2 observations, both showing a small increase of approximately +0.5% per decade for the timespan of the OMI mission (2004-2010). Observations from OMI and the spaceborne lidar CALIOP were used to characterize the around the world transport of an aerosol plume that was released by the intense Australian forest fires of December 2006. The plume crossed the Pacific in 5 days and completed the circumnavigation of the globe in 12 days. Estimates of the plume’s altitude from the OMI cloud retrieval algorithm indicate that the plume was injected into the tropopause region by pyro-convection, triggered by the combination of a passing cold front and the latent heat of the fires. The high altitude of the plume was confirmed by CALIOP that detected the plume at 11-15 km altitude as it passed over South America. Radiative transfer calculations indicate that the underestimation of the OMI plume height in comparison with CALIOP in a later stage of the plume’s transport is caused by photons scattered from lower-lying clouds that outshine the diluted plume. Simulations with TM4 agree best with OMI and CALIOP observations of the plume’s transport when a passive tracer is released at approximately 10 km altitude, to mimic the effect of pyro-convective lofting which is not simulated by the model
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
